Question

The autosomal dominant disease nail-patella syndrome is linked to the ABO blood type locus. The pedigree below shows a family in which some individuals are affected by nail-patella syndrome. The blood type of each individual is given as capital letters above each individual symbol.

А. B — 1 2 АВ 0 ІІ 1 2 A B B B A B B A 1 2 3 4 5 6 7 ою

Can you explain this pattern without a cross-over event? If not in which individual did the cross-over occur?

Answer 1

The I1 individual has A blood type and he is affected. The II1 female is affected and she has AB blood type. Thus, she got A' allele from her father (this A' allele has the nail-patella disease gene) and B allele from her mother. The III1, III5 and III8 individuals also have A blood type and must got the A' allele from their mother (II1).

Thus, it is possible to explain this pattern without a cross over because all the individual who has A' allele (either with blood type A or AB) are affected. As the nail-patella disease gene is present in the A allele, these individuals also got the disease.

In the individual III4, the cross-over is occured because this individual does not have blood type A or AB, rather he has B blood type and got B allele. The crossing over event must be occurred in the individual II1 between A' and B alleles.